Spaster pattern and pulse blanking

ABSTRACT

System and method of photoaltering a region of a material using a pulsed laser beam. The method includes scanning the pulsed laser beam in a first portion of the region with a first pattern, scanning the pulsed laser beam in a second portion of the region with a second pattern, and separating a flap of the material at the region. The system includes a laser, a controller selecting at least first and second patterns, and a scanner operable in response to the controller. The first pattern has a first maximum acceleration associated with the second portion, and the second pattern has a second maximum acceleration associated with the second portion. The second maximum acceleration is less than the first maximum acceleration. The scanner scans the pulsed laser beam from the laser in the first portion with the first pattern and in the second portion with the second pattern.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention is generally related to lasersscanners and more particularly, to systems and methods for scanningpulsed laser beams.

2. Background

Pulsed laser beams include bursts or pulses of light, as implied byname, and have been used for photoalteration of materials, bothinorganic and organic alike. Typically, a pulsed laser beam is focusedonto a desired area of the material to photoalter the material in thisarea and, in some instances, the associated peripheral area. Examples ofphotoalteration of the material include, but are not necessarily limitedto, chemical and physical alterations, chemical and physical breakdown,disintegration, ablation, vaporization, or the like.

One example of photoalteration using pulsed laser beams is thephotodisruption (e.g., via laser induced optical breakdown) of amaterial. Localized photodisruptions can be placed at or below thesurface of the material to produce high-precision material processing.For example, a micro-optics scanning system may be used to scan thepulsed laser beams to produce an incision in the material and create aflap therefrom. The term “scan” or “scanning” refers to the movement ofthe focal point of a pulsed laser beam along a desired path. To create aflap of the material, the pulsed laser beam is typically scanned along apre-determined region (e.g., within the material) in either a spiralpattern or a raster pattern. In general, these patterns are mechanicallysimple to implement (e.g., continuous) and control for a given scan rateand desired focal point separation of the pulsed laser beam.Additionally, these patterns are generally efficient.

Despite these advantages, the spiral or raster pattern may impose limitson the creation of the flap (e.g., due to mechanical restrictions on themicro-optic based scanning system or the like). In general, faster scanrates are desirable but existing laser scanning equipment may lagcommanded laser positions along one axis or both axes and thus, shortenor compress one or more raster scan lines along another axis. Forexample, a circular scan area using a raster pattern may becomeelliptical with faster scan rates. In addition, faster scan rates mayresult in greater accelerations of a mass associated with the scanningsystem, and these greater accelerations complicate control accuracy. Forexample, greater accelerations have been observed while scanning of thecentral region of a spiral pattern (e.g., as the spiral tightens).Greater accelerations have also been observed while scanning theperiphery of a raster pattern (e.g., as the scanning changes directionwith the raster pattern).

Accordingly, it is desirable to provide a system and method for scanninga pulsed laser beam that improves scanning control. More particularly,it is desirable to provide a system and method for scanning a pulsedlaser beam that reduces accelerations during scanning. It is alsodesirable to provide a system and method for creating a flap with apulsed laser beam operating at increased pulse repetition rates whilemaintaining or reducing the acceleration associated with scanning thepulsed laser beam. Additionally, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description and the appended claims, taken inconjunction with the accompanying drawings and the foregoing technicalfield and background.

SUMMARY OF THE INVENTION

The present invention is directed towards photoaltering a material usinga pulsed laser beam. In one embodiment, a method of photoaltering aregion of the material using a pulsed laser beam is provided. The methodincludes scanning the pulsed laser beam in a first portion of the regionwith a first pattern, scanning the pulsed laser beam in a second portionof the region with a second pattern, and separating a flap of thematerial at the region. The first pattern has a first maximumacceleration associated with the region, and the second pattern has asecond maximum acceleration less than the first maximum acceleration.

In another embodiment, a system for photoaltering a region of thematerial is provided. The system includes a laser configured to producea pulsed laser beam, a controller configured to select at least a firstpattern and a second pattern, and a scanner coupled to the controller.The first pattern has a first maximum acceleration associated with thesecond portion, and the second pattern has a second maximum accelerationassociated with the second portion. The second maximum acceleration isless than the first maximum acceleration. The scanner is operable toscan the pulsed laser beam in a first portion of the region with thefirst pattern, and scan the pulsed laser beam in a second portion of theregion with the second pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference numerals refer to similarcomponents:

FIG. 1 is a block diagram of a laser scanner system in accordance withone embodiment of the present invention;

FIG. 2 is a top view of a first compound scan pattern in accordance withone embodiment;

FIG. 3 is a top view of a second compound scan pattern in accordancewith another embodiment;

FIG. 4 is a top view of a third compound scan pattern in accordance withanother embodiment;

FIG. 5 is a top view of a fourth compound scan pattern in accordancewith another embodiment;

FIG. 6 is a top view of a fifth compound scan pattern in accordance withanother embodiment;

FIG. 7 is a top view of a sixth compound scan pattern in accordance withanother embodiment;

FIG. 8 is a top view of a hypotrochoid scan pattern in accordance withanother embodiment;

FIG. 9 is a graph of acceleration versus distance for a compound scanpattern in accordance with one embodiment; and

FIG. 10 is a flow diagram of a method for photoaltering a material inaccordance with one embodiment.

DETAILED DESCRIPTION

The present invention provides systems and method for scanning a pulsedlaser beam that reduces acceleration affects associated therewith.Photoalteration of a material may be accomplished using a pulsed laserbeam that is directed (e.g., via a scanner) at a desired region of thematerial. For example, a pulsed laser beam may be controlled to scan adesired region in the material to produce a flap. To impart at least aportion of this control, software, firmware, or the like, can be used tocommand the actions and placement of the scanner via a motion controlsystem, such as a closed-loop proportional integral derivative (PID)control system. In one embodiment, these systems and methods reduceaccelerations, as well as the acceleration effects associated withscanning a conventional pattern (e.g., a spiral pattern or a rasterpattern), while maintaining a desired scan rate and focal point spatialseparation of the pulsed laser beam. The term “acceleration” is definedherein to be the acceleration of a mass associated with a scanningelement or a scanning system including, but not necessarily limited to,a laser, a laser scanner, scanning mirrors, a scanning chassis, focusingoptics, and any combination thereof. Alternatively, these systems andmethods limit the accelerations associated with scanning theconventional pattern while permitting a faster scan rate of the pulsedlaser beam.

In another embodiment, blanking is incorporated with the scanning toimprove scanning accuracies. The term “blanking” is defined herein to bea prevention of a laser pulse transmission (e.g., via shuttering of thepulsed laser beam). For example, when using a substantially constantlaser pulse rate (e.g., for the pulsed laser beam), blanking is used toavoid altering this laser pulse rate and limit overlap of scanned pulsespots. Additionally, blanking may be used to slow the energy depositionrate into the material. In embodiments incorporating blanking, thepulsed laser beam is prevented from scanning over a previous scan (e.g.,one or more spots associated with one or more focal points of a pulsedlaser beam).

Referring to the drawings, a system 10 for photoaltering a material 12is shown in FIG. 1. The system 10 includes, but is not necessarilylimited to, a laser 14 capable of generating a pulsed laser beam 18, anenergy control module 16 for varying the pulse energy of the pulsedlaser beam 18, a scanner 20, a controller 22, and focusing optics 28 fordirecting the pulsed laser beam 18 from the laser 14 on or within thematerial 12. The controller 22, such as a processor operating suitablecontrol software, is in communication with the scanner 20, the focusingoptics 28, and the energy control unit 16 to direct a focal point 30 ofthe pulsed laser beam along a scan pattern on or in the material 12. Inthis embodiment, the system 10 further includes a beam splitter 26 and adetector 24 coupled to the controller 22 for a feedback controlmechanism of the pulsed laser beam 18. Other feedback methods may alsobe used, including but not necessarily limited to position encoder onthe scanner 20 or the like.

The scanner 20 moves the focal point of the pulsed laser beam 18 inincrements through a desired scan pattern as controlled by thecontroller 22. The step rate at which the focal point is moved isreferred to herein as the scan rate. For example, the scanner 20 canoperate at scan rates between about 10 kHz and about 400 kHz, or at anyother desired scan rate. In one embodiment, the scanner 20 generallymoves the focal point of the pulsed laser beam 18 through the desiredscan pattern at a substantially constant scan rate while maintaining asubstantially constant separation between adjacent focal points of thepulsed laser beam 18. For a given scan pattern or combination of scanpatterns (e.g., a compound scan pattern), the time for completing thescan pattern is inversely proportional to the scan rate. Further detailsof laser scanners are known in the art, such as described, for example,in U.S. Pat. No. 5,549,632, the entire disclosure of which isincorporated herein by reference.

To provide the pulsed laser beam, a chirped pulse laser amplificationsystem, such as described in U.S. Pat. No. RE37,585, may be used forphotoalteration. U.S. Pat. Publication No. 2004/0243111 also describesother methods of photoalteration. Other devices or systems may also beused to generate pulsed laser beams. For example, non-ultraviolet (UV),ultrashort pulsed laser technology can produce pulsed laser beams havingpulse durations measured in femtoseconds. Some of the non-UV, ultrashortpulsed laser technology may be used in ophthalmic applications. Forexample, U.S. Pat. No. 5,993,438 discloses a device for performingophthalmic surgical procedures to effect high-accuracy corrections ofoptical aberrations. U.S. Pat. No. 5,993,438 discloses an intrastromalphotodisruption technique for reshaping the cornea using a non-UV,ultrashort (e.g., femtosecond pulse duration), pulsed laser beam thatpropagates through corneal tissue and is focused at a point below thesurface of the cornea to photodisrupt stromal tissue at the focal point.

Although the system 10 may be used to photoalter a variety of materials(e.g., organic, inorganic, or a combination thereof), the system 10 issuitable for ophthalmic applications in one embodiment. In this case,the focusing optics 28 direct the pulsed laser beam 18 toward an eye(e.g., onto or into a cornea) for plasma mediated (e.g., non-UV)photoablation of superficial tissue, or into the stroma of the corneafor intrastromal photodisruption of tissue. In this embodiment, thesystem 10 may also include a lens (not shown) to change the shape (e.g.,flatten or curve) of the cornea prior to scanning the pulsed laser beam18 toward the eye. The system 10 is capable of generating the pulsedlaser beam 18 with physical characteristics similar to those of thelaser beams generated by a laser system disclosed in U.S. Pat. No.4,764,930, U.S. Pat. No. 5,993,438, or the like.

For example, the ophthalmic laser system 10 can produce an ultrashortpulsed laser beam for use as an incising laser beam. This pulsed laserbeam preferably has laser pulses with durations as long as a fewnanoseconds or as short as a few femtoseconds. For intrastromalphotodisruption of the tissue, the pulsed laser beam 18 has a wavelengththat permits the pulsed laser beam 18 to pass through the cornea withoutabsorption by the corneal tissue. The wavelength of the pulsed laserbeam 18 is generally in the range of about 3 μm to about 1.9 nm,preferably between about 400 nm to about 3000 nm, and the irradiance ofthe pulsed laser beam 18 for accomplishing photodisruption of stromaltissues at the focal point is typically greater than the threshold foroptical breakdown of the tissue. Although a non-UV, ultrashort pulsedlaser beam is described in this embodiment, the pulsed laser beam 18 mayhave other pulse durations and different wavelengths in otherembodiments.

For ophthalmic applications, the scanner 20 may utilize a pair ofscanning mirrors or other optics (not shown) to angularly deflect andscan the pulsed laser beam 18. For example, scanning mirrors driven bygalvanometers may be employed, each scanning the pulsed laser beam 18along one of two orthogonal axes. A focusing objective (not shown),whether one lens or several lenses, images the pulsed laser beam onto afocal plane of the system 10. The focal point of the pulsed laser beam18 may thus be scanned in two dimensions (e.g., the x-axis and they-axis) within the focal plane of the system 10. Scanning along thethird dimension, i.e., moving the focal plane along an optical axis(e.g., the z-axis), may be achieved by moving the focusing objective, orone or more lenses within the focusing objective, along the opticalaxis. In preparing a corneal bed for flap separation, for example, acircular area may be scanned using a scan pattern driven by the scanningmirrors. In another embodiment, a dome-shaped area may be scanned usinga three-dimension movement (e.g., along the x-, y-, and z-axes) drivenby the scanning mirrors. The pulsed laser beam 18 photoalters thestromal tissue by scanning the focal point of the pulsed laser beam 18in a pattern of spots (e.g., based on the scan pattern), thedistribution of which is determined by the pulse frequency, the scanrate, and the amount of scan line separation. Generally, higher scanrates, enable shorter procedure times by increasing the rate at whichcorneal tissue can be photoaltered. For example, the scan rates may beselected from a range between about 1 kHz and about 1 GHz with a pulsewidth in a range between about 300 picoseconds and about 10femtoseconds, although other scan rates and pulse widths may be used.

The system 10 may additionally acquire detailed information aboutoptical aberrations to be corrected, at least in part, using the system10. Examples of such detailed information include, but are notnecessarily limited to, the extent of the desired correction, and thelocation in the cornea of the eye where the correction can be made mosteffectively. The refractive power of the cornea may be used to indicatecorrections. Wavefront analysis techniques, made possible by devicessuch as a Hartmann-Shack type sensor (not shown), can be used togenerate maps of corneal refractive power. Other wavefront analysistechniques and sensors may also be used, such as Tscherning basedaberrometry, ray tracing type wavefront analysis (e.g., Tracey VFA), andspatial skiametry (e.g., Nidek OPD-Scan). The maps of corneal refractivepower, or similar refractive power information provided by other means,such as corneal topographs or the like, can then be used to identify andlocate the optical aberrations of a cornea that require correction.

When the laser 14 is activated, the focal spot 30 of the pulsed laserbeam 18 is selectively moved (e.g., via the scanner 20) along a beampath to photoalter stromal tissue, also referred to herein as scanning.For example, the focal spot 30 of the pulsed laser beam 18 isselectively directed along a predetermined length of the beam path inone reference area. The pulsed laser beam 18 is then redirected throughanother reference area, and the process of photoalteration is repeated.The sequence for directing the pulsed laser beam 18 through individuallyselected reference areas can be varied, and the extent of stromal tissuephotoalteration while the incising laser beam is so directed, can bevaried. Specifically, as indicated above, the amount of photoalterationcan be based on the refractive power map. On the other hand, thesequence of reference areas that is followed during a customizedprocedure will depend on the particular objectives of the procedure.

Scanning may be applied using one or more scan patterns to one or morecombinations of these reference areas. One example of an ophthalmicscanning application is a laser in-situ keratectomy (LASIK) typeprocedure where a flap is cut from the cornea to establishextracorporeal access to the tissue that is to be photoaltered. The flapmay be created using one or more scan patterns of pulsed laser beams. Tocreate the corneal flap, a sidecut is created around a desired perimeterof the flap such that the ends of the sidecut terminate, withoutintersection, to leave an uncut segment. This uncut segment serves as ahinge for the flap. The flap is separated from the underlying stromaltissue by scanning the laser focal point across a resection bed, theperimeter of which is approximately defined by and slightly greater thanthe sidecut. Once this access has been achieved, photoalteration iscompleted, and the residual fragments of the photoaltered tissue areremoved from the cornea. Additionally, the pulsed laser beams may bescanned to customize the incisions, such as for shaped incisions withadvanced edge profiles (e.g., Intralase-enable keratoplasty (IEK)). Inanother embodiment, intrastromal tissue may be photoaltered by thesystem 10 so as to create an isolated lenticle of intrastromal tissue.The lenticle of tissue can then be removed from the cornea to alter thehealed curvature of the cornea and change the corresponding refractiveproperties.

Generally, to create a flap in ophthalmic applications, the pulsed laserbeam 18 is scanned at a substantially constant scan rate whilemaintaining a substantially constant separation between adjacent focalpoints of the pulsed laser beam 18. In one embodiment, the controller 22directs the scanner 20 to scan the pulsed laser beam 18 along a compoundscan pattern that includes two or more scan patterns for the desiredregion of photoalteration (e.g., a resection bed). These scan patternsare preferably selected (e.g., by the controller 22) such that highacceleration regions normally associated with a first scan pattern aresubstituted by a second scan pattern. The second scan pattern isselected such that scanning the second scan pattern in the highacceleration region (i.e., associated with the first scan pattern) isaccomplished with a lower acceleration (e.g., less than a maximumacceleration associated scanning the high acceleration region), whilemaintaining a substantially constant scan rate of the pulsed laser beam18. Although, the compound scan pattern is described with two differentscan patterns, multiple scan patterns may be used.

Additionally, the system 10 may use blanking with portions of one ormore of scan patterns to reduce repetitious scans of the same spot.While scanning a pulsed laser beam normally produces a substantiallycontinuous train of laser pulses, blanking may be incorporated toselectively prevent one or more of the pulses of the pulsed laser beamfrom being scanned over a prior scan spot. For example, while scanning atrain of pulses, the relative position of the scanner 20 (e.g., relativescanning mirror(s) position) associated with each scan spot may berecorded for a particular procedure, timely compared with subsequentlaser pulses. Subsequent laser pulses that might overlap with prior scanspots may then be blanked. In the one embodiment, the relative positionof the scanner 20 may be predicted for each scan spot based on asubstantially constant laser pulse rate for a selected scan pattern andthus, blanking can be pre-determined for subsequent laser pulses.

Some examples of compound scan patterns include a spiral scan patternhaving a central area with a traveling circular scan pattern, asinusoidal scan pattern, a progressive oval scan pattern, or the like.FIG. 2 is a top view of a first compound scan pattern 40 in accordancewith one embodiment. The first compound scan pattern 40 includes aspiral scan pattern 42 and a traveling circular scan pattern 43. Thetraveling circular scan pattern 43 can be produced by scanning multiplecircles while traversing in a linear direction after substantiallycompleting each full circle. A first portion 46 of the travelingcircular scan pattern 43 overlaps the spiral scan pattern 42, and asecond portion 44 of the traveling circular scan pattern 43 occupies acentral region of the first compound scan pattern 40 (e.g., asubstantially circular central area). FIG. 3 is a top view of a secondcompound scan pattern 48 in accordance with another embodiment. Thesecond compound scan pattern 48 includes the spiral scan pattern 42 andthe second portion 44 of the traveling circular scan pattern 43occupying a central region (e.g., a substantially circular central area)of the second compound scan pattern 48, both shown in FIG. 2, but omits(e.g., via blanking) the first portion 46 of the traveling circular scanpattern 43. The traveling circular scan pattern 43 has a substantiallyconstant acceleration associated with the circular shape.

FIG. 4 is a top view of a third compound scan pattern 50 in accordancewith another embodiment. The third compound scan pattern 50 includes thespiral scan pattern 42 shown in FIG. 2 and a sinusoidal scan pattern 53.For simplicity of illustration, the sinusoidal scan pattern 53 travelslinearly along the horizontal axis and is shown in a compressedconfiguration (e.g., compressed along the horizontal axis) having arelatively high frequency appearance but may be elongated or furthercompressed for a selected spot separation. A first portion 54 of thesinusoidal scan pattern 53 overlaps the spiral scan pattern 42, and asecond portion 52 of the sinusoidal scan pattern 53 occupies a centralregion (e.g., a substantially circular central area) of the thirdcompound scan pattern 50 (e.g., a substantially circular central area).The sinusoidal scan pattern 53 is scanned such that the deflections ofthe sinusoidal scan pattern 53 (e.g., the crests of a sine wave) arescanned outside of the central region. In this embodiment, the secondportion 52 of the sinusoidal scan pattern 53 (occupying the centralregion) has a substantially constant acceleration associated with thesubstantially linear segments of the sine wave. FIG. 5 is a top view ofa fourth compound scan pattern 56 in accordance with another embodiment.The fourth compound scan pattern 56 includes the spiral scan pattern 42and the second portion 52 of the sinusoidal scan pattern 53 occupying acentral region (e.g., a substantially circular central area) of thefourth compound scan pattern 56, both shown in FIG. 4, but omits (e.g.,via blanking) the first portion 54 of the sinusoidal scan pattern 53.

FIG. 6 is a top view of a fifth compound scan pattern 58 in accordancewith another embodiment. The fifth compound scan pattern 58 includes thespiral scan pattern 42 shown in FIG. 2 and a progressive oval scanpattern 63 occupying a central region of the fifth compound scan pattern58. The progressive oval scan pattern 63 has a self-overlapping portion62 and a non-overlapping portion 60. The progressive oval scan pattern63 is scanned such that an oval shape (having substantially fixed andspaced apart opposing curved apexes) scan progressively contracts orprogressively expands with each completed pass of the oval shape. Thecurved apexes of the oval shape are associated with lower accelerationsFIG. 7 is a top view of a sixth compound scan pattern 64 in accordancewith another embodiment. The sixth compound scan pattern 64 includes thespiral scan pattern 42 and the non-overlapping portion 60 of theprogressive oval scan pattern 63, both shown in FIG. 6, but omits theself-overlapping portion 62 (e.g., via blanking).

FIG. 8 is a top view of a hypotrochoid scan pattern 66 in accordancewith another embodiment. Referring to FIGS. 2-8, the hypotrochoidpattern 66 can be incorporated with a spiral scan pattern, such as thespiral scan pattern 42, to produce another compound scan pattern. In oneembodiment, the hypotrochoid scan pattern is scanned such that ahypotrochoid shape progressively contracts or progressively expands witheach pass of the hypotrochoid shape. The hypotrochoid scan pattern 66has a central region 68 that may be used when scanning the highacceleration region of the spiral scan pattern (e.g., such as associatedwith the central regions 44, 52, and 60). For example, the hypotrochoidscan pattern 66 may be scanned over a high acceleration region such thatthe central region 68 overlays the high acceleration region. Blankingmay also be used to omit scanning portions of the hypotrochoid scanpattern 66 over the spiral scan pattern. Additionally, blanking can beused to avoid double scanning rhomboid shaped self-overlapping regions69 within the hypotrochoid pattern 66. The curved apexes of thehypotrochoid shape are associated with lower accelerations.

FIG. 9 is a plot 70 of acceleration versus distance for a compound scanpattern illustrating a reduction in acceleration for a central region ofthe compound scan pattern in accordance with one embodiment. Thecompound scan pattern includes a first scan pattern and a second scanpattern. Referring to FIGS. 2-9, the compound scan pattern of thisembodiment can be any one of the compound scan patterns 40, 48, 50, 56,58, and 64 or other compound scan patterns based on a combination of aspiral scan pattern and a second scan pattern (e.g., the hypotrochoidscan pattern 68 or the like). For example, the spiral scan pattern 42 islocated in the periphery of the compound scan pattern, and the secondscan pattern is located in the central region of the compound scanpattern, such as the central regions 44, 52, and 60. In this embodiment,the central region radially extends outward from the center of thespiral scan pattern 42 to about 1000 μm.

The compound scan pattern is preferably scanned at a substantiallyconstant scan rate to simplify control, for example. As scanning (i.e.,the spiral scan pattern) approaches the central region, the accelerationgenerally increases. For example, as the spiral scanning becomesnarrower, the acceleration movement of the scanning element or scanningsystem significantly increases. Scanning of the second scan patternpreferably initiates when the acceleration associated with scanning thefirst scan pattern reaches a pre-determined maximum limit. This limitmay be selected based on one or more factors, such as mechanicallimitations of the system 10, an overall procedure time to complete thescanning (e.g., of the entire scan region), the scan rate, historicaldata corresponding to a departure from a desired scanning quality, flapquality, uniformity of scan spot separation or scan line separation, orthe like.

In this embodiment, the second scan pattern begins at about 1000 μm fromthe center of the scan region and has a substantially constantacceleration associated therewith. For example, each of the travelingcircular scan pattern 43, the sinusoidal scan pattern 53, and theprogressive oval scan pattern 63 has a maximum acceleration associatedwith scanning the respective pattern in the central regions 44, 52, and60, respectively, and these maximum accelerations are each less than themaximum acceleration associated with scanning the spiral scan pattern 42in the same region. As best shown in FIG. 9, the second scan pattern hasa maximum acceleration (e.g., about 7.3 g), associated with the scanningthereof in a central region of a compound scan pattern, that issubstantially less than the maximum acceleration of the spiral pattern(e.g., about 9.4 g) associated with scanning the spiral scan patternbeyond the central region, or in the periphery, of the compound scanpattern (e.g., at about 1000 μm). The maximum acceleration of the secondscan pattern would also be substantially less than any extrapolatedmaximum acceleration (not shown) associated with scanning of the spiralscan pattern in the central region. The compound scan pattern thusreduces acceleration effects normally associated with solely scanning aconventional spiral scan pattern.

In effect, the first scan pattern (e.g., the spiral scan pattern 42) isscanned in a first sub-region (e.g., the periphery of a desired scanregion), and the second scan pattern (e.g., the traveling circular scanpattern 44, the sinusoidal scan pattern 52, the progressive oval scanpattern 60, or the like) is scanned in a second sub-region (e.g., acentral area) of the desired scan region. For example, the travelingcircular scan pattern 44, sinusoidal scan pattern 52, or progressiveoval scan pattern is scanned in a substantially circular central region44, 52, or 60, respectively, that corresponds to the high accelerationregion of the spiral pattern. While the compound scan patterns 40, 48,50, 56, 58, and 64 are illustrated as being scanned within asubstantially circular scan region, the scan region may take a varietyof shapes and thus, other compound scan patterns may be used to scandifferent shaped scan regions.

For most of the compound scan patterns, the second scan patternprogresses from a starting point at one location of the secondsub-region to a completion point at a different location of the secondsub-region. An amount of time (e.g., a time lag) is expended to scan thesecond sub-region with the second scan pattern. During this timer thematerial (e.g., the cornea) may move, expand, stretch, or relax due tothe biomechanical change induced in the material by the pulsed laserbeam 18. The greater the time expended to cease the progression of anincision and finish the incision with the subsequent compound segment,the greater the effect associated with this movement. For example,corneal tissue movement can manifest as a ridge at the boundary ofcompound segments, which may adversely affect the optical imagingproperties of the treated corneal tissue. Uninterrupted compoundpatterns may be selectively applied to alleviate this time lag.

With uninterrupted compound patterns, the scanning progression issubstantially continuous from completing of the first sub-region scan toinitiating the second sub-region scan. In one embodiment, the scanningprogression within one sub-region does not cease at any time or at anypart of the sub-region boundary. The maximum amount of time lag is onthe order of one turn-around of the pulsed laser beam (e.g., a fewmilliseconds), instead of a much longer time lag associated withinterrupted compound patterns. The hypotrochoid scan pattern 66 and theprogressive oval scan pattern 63 are examples of uninterrupted patterns,and the traveling circular scan pattern 43 is an example of aninterrupted pattern.

By minimizing or eliminating higher accelerations of the scanningsystem, scanning accuracy of the pulsed laser beam is increased.Additionally, vibrations that may be associated with such higheraccelerations can be reduced to improve reliability of the scanningsystem. By minimizing or eliminating higher accelerations, the pulsedlaser beam may operate with higher laser repetition rates and thus,reduce procedure times (e.g., associated with creating a desired scanregion).

In general, the temporal sequence of various sub-patterns is not relatedto the overall limit on the maximum acceleration or procedure time. Thetemporal sequence is preferably determined by other considerations. Oneexample of such considerations is to provide an exhaust for gas that maybe formed during plasma created laser incisions (e.g., U.S. Pat. No.6,676,653) for the sub-pattern that is first created in the temporalsequence.

FIG. 10 is a flow diagram of a method 100 for photoaltering a region ofa material using a pulsed laser beam in accordance with one embodiment.The pulsed laser beam is scanned in a first portion of the region with afirst pattern, as indicated at 105. The pulsed laser beam is scanned ina second portion of the region with a second pattern, as indicated at110. A flap of the material is separated at the region, as indicated atstep 115.

In one embodiment, the first pattern has a maximum scan accelerationassociated with scanning the first pattern in the region, the secondpattern has a maximum scan acceleration associated with scanning thesecond pattern in the second portion, and the maximum scan accelerationof the second pattern is less than the maximum scan acceleration of thefirst pattern. In another embodiment, the first pattern has a maximumscan acceleration associated with scanning the first pattern in thesecond portion, the maximum scan acceleration of the second pattern isassociated with scanning the second pattern in the second portion, andthe maximum scan acceleration of the second pattern is less than themaximum scan acceleration of the first pattern.

At least some of the second portion of the region may overlap with atleast some of the first portion of the region to form a third portion ofthe region. In this embodiment, the pulsed laser beam is blanked whilescanning the pulsed laser beam with the second pattern in the thirdportion. The region may also have a periphery. In this embodiment, thepulsed laser beam is scanned in the first portion in a spiral patternbeginning from the periphery of the region.

The pulsed laser beam may be scanned in an uninterrupted compoundpattern. Referring to FIGS. 2-10, the pulsed laser beam may be scannedin the second portion in the traveling circular scan pattern 43, thesinusoidal scan pattern 53, or the progressive oval scan pattern 63, forexample. Additionally, the pulsed laser beam may be blanked when thetraveling circular pattern 43 overlaps the spiral scan pattern 42, whenthe sinusoidal scan pattern 53 overlaps the spiral scan pattern 42, orwhen the progressive oval scan pattern 63 overlaps itself.

Referring to FIGS. 1 and 10, the system 10 may be used to photoalter thematerial using a pulsed laser beam 18. For example, the pulsed laserbeam 18 may be scanned (with either the first or second pattern) at arate between about 1 kHz and about 1 GHz, with a pulsed energy of about800 nJ/pulse, with a pulse width of between about 300 picoseconds andabout 10 femtoseconds, and/or at a wavelength between about 400 nm toabout 3000 nm. Additionally, the pulsed laser beam 18 may be scanned ata sub-surface depth of the material 12.

Thus, systems and methods of photoaltering a material with a pulsedlaser beam are disclosed that reduce accelerations associated withscanning the pulsed laser beam. While embodiments of this invention havebeen shown and described, it will be apparent to those skilled in theart that many more modifications are possible without departing from theinventive concepts herein. The invention, therefore, is not to berestricted except in the spirit of the following claims.

1. A method of photoaltering a region of a material using a pulsed laserbeam, the region having a first portion and a second portion, the methodcomprising the steps of: scanning the pulsed laser beam in the firstportion of the region with a first pattern, the first pattern having afirst maximum scan acceleration associated with the region; scanning thepulsed laser beam in the second portion of the region with a secondpattern, the second pattern having a second maximum scan accelerationless than the first maximum scan acceleration; and separating a flap ofthe material at the region.
 2. The method of claim 1, wherein the stepof scanning the pulsed laser beam in the first portion comprisesscanning the pulsed laser beam with the first pattern having the firstmaximum scan acceleration, the first maximum scan accelerationassociated with the second portion, and wherein the step of scanning thepulsed laser beam in the second portion comprises scanning the pulsedlaser beam with the second pattern having the second maximum scanacceleration, the second maximum scan acceleration associated with thesecond portion.
 3. The method of claim 1, wherein at least some of thesecond portion of the region overlaps at least some of the first portionof the region to form a third portion of the region, and wherein thestep of scanning the pulsed laser beam in the second portion comprisesblanking the pulsed laser beam while scanning the pulsed laser beam withthe second pattern in the third portion.
 4. The method of claim 1,wherein the region has a periphery, and wherein the step of scanning thepulsed laser beam in a first portion comprises scanning the pulsed laserbeam in a spiral pattern beginning from the periphery of the region. 5.The method of claim 1, wherein the step of scanning the pulsed laserbeam in the first portion comprises scanning the pulsed laser beam at arate between about 1 kHz and about 1 GHz, and wherein the step ofscanning the pulsed laser beam in the second portion comprises scanningthe pulsed laser beam at a rate between about 30 MHz and about 1 GHz. 6.The method of claim 1, wherein the step of scanning the pulsed laserbeam in the first portion comprises scanning the pulsed laser beam witha pulse energy of about 800 nJ/pulse, and wherein the step of scanningthe pulsed laser beam in the second portion comprises scanning thepulsed laser beam with a pulse energy of about 800 nJ/pulse.
 7. Themethod of claim 1, wherein the step of scanning the pulsed laser beam inthe first portion comprises scanning the pulsed laser beam with a pulsewidth of between about 300 picoseconds and about 10 femtoseconds, andwherein the step of scanning the pulsed laser beam in the second portioncomprises scanning the pulsed laser beam with a pulse width betweenabout 300 picoseconds and about 10 femtoseconds.
 8. The method of claim1, wherein the step of scanning the pulsed laser beam in the firstportion comprises scanning the pulsed laser beam at a wavelength betweenabout 400 nm to about 3000 nm, and wherein the step of scanning thepulsed laser beam in the second portion comprises scanning the pulsedlaser beam at a wavelength between about 400 nm to about 3000 nm.
 9. Themethod of claim 1, wherein the step of scanning the pulsed laser beam inthe second portion comprises scanning the pulsed laser beam in anuninterrupted compound pattern.
 10. The method of claim 9, wherein thestep of scanning the pulsed laser beam in an uninterrupted compoundpattern comprises scanning the pulsed laser beam in a hypotrochoidpattern.
 11. The method of claim 1, wherein the step of scanning thepulsed laser beam in the second portion comprises scanning the pulsedlaser beam in a traveling circular pattern.
 12. The method of claim 11,wherein the step of scanning the pulsed laser beam in a travelingcircular pattern comprises blanking the pulsed laser beam when scanningthe traveling circular pattern overlaps the spiral pattern.
 13. Themethod of claim 1, wherein the step of scanning the pulsed laser beam inthe second portion comprises scanning the pulsed laser beam in asinusoidal pattern.
 14. The method of claim 13, wherein the step ofscanning the pulsed laser beam with a sinusoidal pattern comprisesblanking the pulsed laser beam when scanning the sinusoidal patternoverlaps the spiral pattern.
 15. The method of claim 1, wherein the stepof scanning the pulsed laser beam in the second portion comprisesscanning the pulsed laser beam in a progressive oval pattern.
 16. Themethod of claim 15, wherein the step of scanning the pulsed laser beamin a progressive oval pattern comprises blanking the pulsed laser beamwhen scanning the progressive oval pattern overlaps itself.
 17. Themethod of claim 1, wherein the step of scanning the pulsed laser beam inthe first portion comprises scanning the pulsed laser beam at asub-surface depth of the material, and wherein the step of scanning thepulsed laser beam in the second portion comprises scanning the pulsedlaser beam at the sub-surface depth of the material.
 18. A system forphotoaltering a region of a material, the region having first and secondportions, the system comprising: a laser configured to produce a pulsedlaser beam; a controller configured to select at least a first patternand a second pattern, the first pattern having a first maximumacceleration associated with the second portion, the second patternhaving a second maximum acceleration associated with the second portion,the second maximum acceleration being less than the first maximumacceleration; and a scanner operable in response to the controller to:scan the pulsed laser beam in the first portion of the region with thefirst pattern; and scan the pulsed laser beam in the second portion ofthe region with the second pattern.
 19. The system of claim 18, whereinthe first pattern is a spiral pattern.
 20. The system of claim 18,wherein the second pattern is selected from a group consisting of atraveling circular pattern, a sinusoidal pattern, and a progressive ovalpattern.
 21. The system of claim 18, wherein the region is asubstantially circular region having a central portion, and wherein thesecond portion is the central portion.
 22. The system of claim 18,wherein the controller is further configured to select a compoundpattern comprising the first pattern and the second pattern.
 23. Thesystem of claim 18, wherein the pulsed laser beam has a pulse frequencyselected from a range of about 1 kHz to about 1 GHz.
 24. The system ofclaim 18, wherein the first pulse energy is less than or equal to about800 nanojoules.
 25. The system of claim 18, wherein the material has asurface layer, and wherein the laser is further configured to scan thepulsed laser beam along the region of the material to at least partiallyseparate the surface layer from the material, the region underlying thesurface layer of the material.